The present invention relates generally to coherent demodulators for a frequency shift keyed (FSK) signal and more particularly to coherent detectors for a type of frequency shift keyed signal commonly referred to as a minimum shift keyed (MSK) signal that may be advantageously utilized in data communication systems.
MSK is particularly well suited for use in radio communication systems since the spectral energy is more easily contained within the limited bandwidth available than other binary frequency or phase-shift keying modulations. Frequency shift key (FSK) signals result when binary data modulate a carrier by shifting its frequency between two distinct levels known as mark and space tones or frequencies depending on the value of a binary digit or bit being transmitted. A MSK signal is a continuous-phase-frequency-shift-keying (CPFSK) system with a modulation index of one-half. Therefore, the upper and lower tones are frequency shifted by one-fourth of the transmitter data bit rate above and below the carrier frequency. MSK is a constant amplitude signal allowing the use of well known FM transmitter and receiver techniques.
A type of frequency modulation called tamed frequency modulation for binary data transmission employs an angle-modulated carrier signal of substantially constant amplitude and continuous phase, whose continuous phase in each symbol period T changes by an amount, expressed in radians from the series -.pi./2, -.pi./4, 0, .pi./4, .pi./2. The tamed frequency modulated signal also is a binary data modulated signal of constant amplitude allowing the use of well known FM transmitter and receiver techniques.
The binary signals can be detected optimally by means of coherent signal detectors which advantageously exploit the unique characteristics of both the MSK signal and tamed frequency modulation. Coherent signal detectors use the phase of the signaling tones to detect the transmitted bits. As such, this type of demodulator usually consists of two parts, a device which extracts the bit clock and the phase of the signaling tones, and a detector which uses the extracted signal phase.
One known coherent demodulator employs two phase-locked loops (PLL), that are operated in parallel and are intended to lock onto two separate frequencies to extract the mark and space frequencies. The PLL's are susceptible to interference and can lock onto the wrong frequency. Also, the PLL when locked to the desired frequency can unlock due to interference, and the PLL's cannot track large phase changes due to a fading field. A frequency synthesizer and control logic circuitry has been used with the two PLL's to provide more reliable operation, but such a coherent demodulator has a high cost. Other known more reliable coherent demodulators also require complex circuitry and consequently also have a high cost.